Linearly chirped signal is one of the signal waveforms frequently employed by a modern radar system, with that of large bandwidth ones capable of elevating radar range resolution. Electronics based prior art generators of arbitrary waveforms generally have comparative narrow bandwidth due to electronic bottlenecks. In comparison, a photon-based linearly chirped signal is capable of providing much larger bandwidth than that based on traditional electronics, with the further advantage of low loss and resistance to electromagnetic interference, and thus may be employed for furtherance of radar capacity.
Methods for generating photon-based broadband linearly chirped signal are mostly based on spatial light, phase modulation or polarization modulation, or that of wavelength-time mapping and of time domain pulse reshaping (See, J. Yao, “Photonic generation of microwave arbitrary waveforms”, Optics Communications, vol. 284, no. 15, pp. 3723-3736, 2011). Among the methods, the method of wavelength-time mapping is more attractive owing to its good tunability. Canadian researchers have proposed a method based on fiber Bragg grating and wavelength-time mapping (See R. Ashrafi, Y. Park, and J. Azaña, “Fiber-based photonic generation of high-frequency microwave pulses with reconfigurable linear chirp control,” IEEE Transactions on Microwave Theory and Techniques, Vol. 58, no. 11, pp. 3312-3319, 2010.). The shortcoming of this scheme is that the tuning of the center frequency is at the price of decrease of pulse duration, with a further disadvantage of incapacity of easy and flexible control of bandwidth. The Tsinghua University proposes a method of beat frequency of continuous light and mode-locked laser (See H. Gao, C. Lei, M. Chen, F. Xing, H. Chen, and S. Xie, “A simple photonic generation of linearly chirped microwave pulse with large time-bandwidth product and high compression ratio,” Optics Express, vol. 21, no. 20, pp. 23107-23115, 2013.). The shortcoming thereof is that use of two unrelated independent lasers results in instability of the generated signal.